Field of the Invention
The present invention relates, in general, to spacer grids used for supporting fuel rods in a nuclear fuel assembly of a nuclear reactor and, more particularly, to a lips-type multi-purposed spacer grid used in such nuclear fuel assemblies and designed to support fuel rods by springs which are in contact with the fuel rods at equiangular curved contact surfaces having areas larger than those of springs of conventional spacer grids, thus enhancing the fuel rod support performance of spacer grids and accomplishing desired soundness of the spacer grids when the spacer grids support the fuel rods in a nuclear fuel assembly. The spacer grid according to the present invention also has a structure designed such that load applied to the springs is distributed to the entire structure of the spacer grid, and the coolant deflecting area of the spacer grid is increased by mixing blades provided at the upper part of the spacer grid, and the spacer grid has a structure capable of effectively filtering the debris moving along with the coolant.
Description of the Prior Art
FIG. 1 is a perspective view, showing the construction of a conventional nuclear fuel assembly.
In each spacer grid 7 of the nuclear fuel assembly 1, springs and dimples support a plurality of elongated nuclear fuel rods 8 and a plurality of guide tubes 5 so as to maintain the arrangement of the nuclear fuel rods 8 placed at regular intervals in the nuclear fuel assembly 1. That is, the springs and dimples maintain the regular intervals between the nuclear fuel rods 8 while preventing impact-induced deformation of the nuclear fuel rods 8 and the guide tubes 5, thus reliably defining passages for coolant in the fuel assembly 1 and allowing the coolant to effectively cool the fuel rods 8 in a reactor core. A plurality of mixing blades are attached to the upper edges of intersecting strips of each spacer grid 7 so as to mix the thermally imbalanced coolant within the nuclear fuel assembly 1. In addition, the spacer grids 7 may be designed to have a structure capable of effectively filtering debris from the coolant.
The coolant mixing function, the wear and the debris filtering function of the spacer grids 7 are recognized as important factors in development of nuclear fuel assemblies of high burn-up and zero defects. In order to develop the nuclear fuel assemblies of high burn-up and zero defects, it is necessary to enhance the thermal efficiency of the nuclear fuel assemblies. The thermal efficiencies of the nuclear fuel assemblies may be enhanced by improving the flow characteristics of coolant around the fuel rods.
The improvement in the flow characteristics of coolant around the fuel rods may be effectively accomplished by a change of the structure of a spacer grid. That is, the thermal mixing of coolant may be improved by attachment of mixing blades to the spacer grid, a change of the shape of the mixing blades and/or defining appropriately designed coolant channels in the spacer grid. However, the above-mentioned methods of enhancing the thermal efficiencies of nuclear fuel assemblies also generate turbulences in coolant flowing around the fuel rods, and the turbulences of the coolant undesirably cause to vibrate the elongated, parallel, closely spaced fuel rods within the nuclear fuel assembly. When the fuel rods are so vibrated over a lengthy period of time, the claddings of the fuel rods are repeatedly and frictionally abraded at their contact parts at which the fuel rods are brought into contact with the springs and dimples of the spacer grids. The claddings are thus reduced in their thickness so as to be finally perforated at the contact parts. Such an abrasion of the fuel rods is so-called xe2x80x9cfretting wear of fuel rodsxe2x80x9d in the art.
When the spacer grids 7 are exposed to neutron irradiation in the reactor core for a lengthy period of time, the material characteristics of the springs of the spacer grids 7 are changed, and the springs gradually lose their elasticity. The springs in such a case fail to stably and steadily support the fuel rods 8, and so allow the fuel rods 8 to vibrate. The claddings of the fuel rods 8 are thus abraded at the contact parts at which the fuel rods are brought into contact with the springs and dimples of each spacer grid 7, so that the fretting wear of the fuel rods 8 occurs. There have been a lot of reports of leakage of radioactive materials from fuel rods due to perforation of the claddings of the fuel rods 8 caused by the fretting wear. In the art, it has been recognized that the fretting wear of the fuel rods is greatly affected by the design of the spacer grid, including the shapes of contact parts between the fuel rods and the spacer grids.
In the conventional spacer grid 7, the fuel rods 8 are in contact with the springs and dimples in a manner of point contact or linear contact mostly. Of the two contact manners, the linear contact manner confers higher resistance to flow-induced vibration and abrasion of fuel rods, in comparison with the point contact manner, so that the linear contact manner more effectively protects the fuel rods 8 from fretting wear. That is, when the contact surface areas between the fuel rods 8 and the springs and dimples of the spacer grid 7 are increased under the condition that the spring force is not changed, the peak value of contact pressure at the contact parts is reduced such that the fretting wear of the fuel rods 8 is diminished.
In addition, during a process of designing the springs of spacer grids, it is necessary to preset the spring force of each spacer grid while considering expected irradiation-induced changes in the material characteristics of the springs. When the spring force is too low, the springs may fail to accomplish desired soundness of a spacer grid when the spacer grid supports fuel rods. When the spring force is too high, the springs may impose excessive frictional force on the claddings of fuel rods, thus scratching the claddings to damage the claddings during a process of installing the fuel rods in the spacer grid while producing a nuclear fuel assembly. In addition, the springs having such excessively high spring force may cause the fuel rods to be undesirably bent when the fuel rods are elongated due to irradiation-induced growth during an operation of a nuclear reactor. A desired fuel rod support soundness of a spacer grid 7 is accomplished when the springs and dimples of the spacer grid 7 have appropriate spring force.
The power distribution of the fuel rods 8 in the reactor core are unevenly distributed, so that the temperature of coolant flowing around higher power fuel rods 8 is relatively high. When the coolant temperature reaches close to the saturation temperature, an excessive amount of bubbles may be generated at a certain area of the cladding of a fuel rod 8 as much as to cover the cladding, so that the bubble crowding remarkably reduce heat transfer from fuel rod to coolant. In such a case, the temperature of the local area of the cladding covered with the bubbles is quickly increased, so that the cladding or the uranium pellets in the fuel rod 8 may be over-heated up to a melting temperature.
In an effort to suppress the possibility of the above-mentioned problem, the spacer grid 7 is designed to forcibly mix the coolant flowing around the fuel rods 8 of the nuclear fuel assembly 1, thus creating a uniform temperature distribution of the coolant in the nuclear fuel assembly 1 as for as possible and thus enhancing the heat transfer rate of the claddings of the fuel rods 8. The spacer grid 7 thus restricts critical nucleate boiling of the coolant and allows the nuclear reactor to be safely operated for a desired lengthy period of time. In order to accomplish the above-mentioned function, mixing blades as a means for increasing the heat transfer efficiency between the fuel rods 8 and the coolant are attached to the spacer grid 7, as a fuel rod support means.
In the art, coolant mixing effect in a nuclear fuel assembly has been accomplished as follows. In a first method, the coolant mixing may be accomplished by strong vortexes generated in coolants flowing through the coolant passages defined among fuel rods. Due to the strong vortexes, the coolants flowing through the coolant passages of the spacer grid are effectively mixed with each other to accomplish a uniform temperature distribution of coolant in the fuel assembly. Second, swirls flow may be generated in the coolants flowing through the spacer grid, so that the coolant having higher density is centrifugally moved toward the surfaces of the claddings, while the bubbles having lower density are centripetally moved toward the centers of the swirls. The second method using the swirls thus prevents a reduction in the heat transfer caused by the bubbles covering the claddings of the fuel rods, so that the fuel rod cooling performance of the nuclear fuel assembly is enhanced.
Of the two methods, the second method using the swirls is more preferable in that the swirls are more slowly fade out after passing through a spacer grid 7, in comparison with the turbulent vortexes used in the first method, and, therefore, the spacer grids proposed in recent years have been designed on the basis of the second method using the swirls. When the mixing blades are attached to the spacer grid, an increase of pressure loss is caused in the nuclear fuel assembly. Such additional pressure loss in the nuclear fuel assembly increases the hydraulic load imposed on a pump, thus reducing the coolant flowing in the nuclear fuel assembly. Therefore, when attaching the mixing blades to each spacer grid of a nuclear fuel assembly, it is necessary to design the mixing blades so as to minimize the pressure loss in the nuclear fuel assembly under the same projected area of the mixing blades.
In recent years, a debris filtering and/or capturing device is also required at a nuclear fuel assembly in an effort to remove debris from coolant and thereby protect the fuel assembly from damage caused by the debris. A variety of debris, such as small-sized bolts or wire pieces, are moved quickly along with coolant in a reactor core, thus sometimes seriously damaging the fuel rods. In order to solve the problem by filtering the coolant, a filtering device may be mounted to a coolant passage in bottom nozzle 3, or a debris filtering spacer grid specifically designed to filter the debris. In such a case, the filtering spacer grid is designed to minimize its interference with the flow of coolant and capture the debris removed from the coolant in the spacer grid, in place of having a fuel rod support function accomplished by the springs and dimples.
In accordance with an extension of fuel life cycle due to provision of nuclear fuel assemblies of high burn-up and zero defects, the quantity of neutrons irradiated to the spacer grids is increased, so that the problem of irradiation-induced changes in the material characteristics of springs, resulting in a reduction in the spring force, becomes worse. Therefore, in order to provide nuclear fuel assemblies with zero defects in terms of fretting wear, it is necessary to provide a spacer grid having a mechanism capable of compensating for the reduction in the spring force and effectively supporting the fuel rods without allowing flow-induced vibration of the fuel rods.
In some high burn-up nuclear fuels, the level of nuclear fuel enrichment may be increased. The increased nuclear fuel enrichment results in much higher power peak than an average fuel rods. Under this situation, the enhanced heat transfer rate from the claddings may become more important considerably. Therefore, it is necessary to provide an advanced spacer grid having a fuel rod cooling performance higher than that of the conventional spacer grid. In addition, it is also necessary to provide a spacer grid capable of solving the problem of excessive pressure loss caused by mixing blades and preventing damage to fuel rods due to debris.
Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a lips-type multi-purposed spacer grid for nuclear fuel assemblies, which allows fuel rods to be in contact with springs and dimples of the spacer grid in an equiangular surface contact manner, different from conventional spacer grids, so that contact force generated at equiangular contact surfaces between the fuel rods and the springs and dimples is evenly distributed to the contact surfaces, thus reducing possible fretting wear of the fuel rods due to flow-induced vibration of the fuel rods, and which enlarges the allowable elastic range of the springs, thus allowing the springs to soundly support the fuel rods by using residual spring force.
Another object of the present invention is to provide a lips-type multi-purposed spacer grid for nuclear fuel assemblies, which has a structure designed such that load applied to the springs in longitudinal and latitudinal directions is distributed to the entire structure of the spacer grid, thus reducing peak stress at the contact surfaces between fuel rods and the springs, and which soundly supports the fuel rods regardless of any directional external force applied to the fuel rods or a variation in the pressure field generated in a nuclear fuel assembly, and which has mixing blades provided at the upper edge of dimples, thus minimizing pressure loss and flow interference caused by conventional mixing blades that have been provided on typical spacer grids to generate vortexes or swirls in coolant, and the mixing blades of which each have a spoon-shaped curve and are sized twice as large as conventional mixing blades, so that the coolant deflecting surfaces and the coolant mixing effect of the mixing blades are maximized, and which minimizes pressure loss of the nuclear fuel assembly by controlling the angle of the mixing blades and reducing the number of the mixing blades by a half compared to the conventional spacer grid, so that the fuel rod cooling efficiency is improved, and which has arc-shaped edges at the springs and dimples to define gaps between the springs and dimples, so that lateral passages for the coolant are formed by the gaps between the arc-shaped edges of the springs and dimples and debris from the coolant are captured at the gaps between the springs and dimples, and which thus minimizes damage to the fuel rods due to such debris.
In order to accomplish the above objects, the present invention provides a lips-type multi-purposed spacer grid fabricated by a plurality of two types of inner strips intersecting each other to form a plurality of unit cells, and used for supporting a plurality of fuel rods in a nuclear fuel assembly such that one fuel rod is supported within one unit cell, each of the two types of inner strips being fabricated by integrating a plurality of unit strip parts into a linear strip, and having a plurality of notches at junctions of the unit strip parts such that each notch vertically extends downward or upward, wherein each of the unit strip parts comprises a frame used as a support frame of the unit strip part, at least one water strider-type spring, and upper and lower dimples. The frame comprises two support columns vertically disposed in parallel while being spaced apart from each other at a predetermined interval, and two support beams horizontally extending between the two support columns at upper and lower positions to connect the two support columns to each other and define a middle opening between the support beams and the support columns. The water strider-type spring is provided in the middle opening of the frame while being projected in a direction from a vertical surface formed by the frame, and comprises an equiangular curved part axially formed along the spring and having a predetermined width while being curved within a direction of the width at a radius of curvature corresponding to that of an external surface of each fuel rod, two side extensions extending outward in opposite directions from both sides of the equiangular curved part to a predetermined width while being bent at a predetermined angle, and four spring legs diagonally extending from upper and lower corners of the two side extensions such that the four spring legs are connected to inside edges of the two support columns at four corners of the middle opening of the frame. The upper and lower dimples are provided at positions above and under the water strider-type spring while being projected from the vertical surface formed by the frame in a direction opposed to the projecting direction of the water strider-type spring. The upper dimple is curved along a lower edge thereof to form an arc-shaped lower edge, and the lower dimple is curved along an upper edge thereof to form an arc-shaped upper edge. Each of the upper and lower dimples comprises a curved dimple part axially formed along each dimple and having a predetermined width while being curved within a direction of the width at a radius of curvature corresponding to that of an external surface of each fuel rod, two side dimple extensions extending outward in opposite directions from both sides of the curved dimple part to a predetermined width while being curved at a predetermined angle.
In the lips-type multi-purposed spacer grid, upper and lower edges of the water strider-type spring are curved to form arc-shaped edges which are symmetrical with respect to a horizontal axis of the water strider-type spring.
In an embodiment, one water strider-type spring comprised of a long equiangular curved part and two long side extensions is provided in the middle opening of the frame.
In another embodiment, two water strider-type springs, each comprising a short equiangular curved part and two short side extensions, are provided at upper and lower portions inside the middle opening of the frame.
In the spacer grid, a mixing blade extends upward to a predetermined length from a side of an upper edge of the upper dimple.